1. Field of the Invention
This invention relates generally to drill bits for drilling into subterranean formations and methods of manufacturing the same, wherein such bits include at least one surface to which formation material exhibits relatively low-adhesion, the low-adhesion surface being effected by coating, plating, or otherwise treating that portion of the bit such as by mechanical or thermal processing.
2. State of the Art
Rotary-type drill bits include both rotary drag bits and roller-cone bits. Typically, in a rotary drag bit, fixed cutting elements made of natural diamond or polycrystalline diamond in the form of polycrystalline diamond compacts (PDCs) are attached to the face of the drill bit, either as freestanding, unbacked cutters or, where suitably configured, mounted or a stud, cylinder or other carrier. The cutters on the bit face are typically adjacent to waterways or fluid courses extending to passageways or xe2x80x9cjunk slotsxe2x80x9d formed in the side or gage surface of the bit body above the bit face (as the bit is oriented for drilling) to allow drilling fluid with entrained material (cuttings) that has been cut from the formation to pass upwardly around the bit and into the borehole thereabove.
In a roller-cone arrangement, the bit typically has three cones, each independently rotatable with respect to the bit body supporting the cones through bearing assemblies. The cones carry either integrally formed teeth or separately formed inserts that provide the cutting action of the bit. The spaces between the teeth or inserts on the cones and between the legs of the bit to which the cones are mounted provide a passage for drilling fluid and formation cuttings to enter the borehole above the bit.
When drilling a hole with prior art drill bits, the cuttings may adhere to, or xe2x80x9cball upxe2x80x9d on, the surface of the drill bit. The cuttings thus tend to accumulate on the cutting elements and the surfaces of the drill bit and collect in any void, gap or recess created between the various structural components of the bit. This phenomenon is particularly enhanced in formations that fail plastically, such as certain shales, mudstones, siltstones, limestones and other ductile formations, the cuttings from which may become mechanically packed in the aforementioned voids, gaps or recesses on the drill bit exterior. In other cases, such as when drilling certain shale formations, the adhesion between a bit surface and the formation cuttings is most probably, or in many instances, caused by a chemical bond. When the surface of a bit becomes water wet in such formations, the bit surface and clay layers of the shale share common hydrogen electrons. A similar sharing of electrons is present between the individual sheets of the shale itself A result of this sharing of electrons is an adhesive-type bond between the shale and the bit surface. Adhesion between the formation cuttings and the bit surface may also occur when the charge of the bit face is opposite the charge of the formation, the oppositely charged formation particles tending to adhere to the surface of the bit. Moreover, particles of the formation may actually be compacted onto exterior surfaces of the bit or mechanically bonded into pits or trenches etched into the bit by erosion and abrasion during the drilling process.
Attempts have been made to alleviate the aforementioned electrical charge-induced adhesion tendencies as disclosed in U.S. Pat. Nos. 5,330,016 and 5,509,490 and in two IADE/SPE papers respectively referenced as IADC/SPE 23870, Roy et al., xe2x80x9cPrevention of Bit Balling in Shales; Some Preliminary Resultsxe2x80x9d and IADC/SPE 35110, Smith et al., xe2x80x9cSuccessful Field Application of an Electro-Negative xe2x80x98Coatingxe2x80x99 to Reduce Bit Balling Tendencies in Water Based Mud.xe2x80x9d
If cuttings become stuck to the surface of the drill bit, subsequent cuttings are not allowed to simply slide along the surface of the cutters and through the junk slots. The subsequent cuttings must, in effect, slide over formation material already attached to the surface of the bit. Thus, a shearing force is created between the cuttings stuck to the bit and subsequent cuttings. As a result, much greater frictional forces between the drill bit and the formation are produced, which forces may result in a reduced rate of penetration and result in further accumulation of cuttings on the bit.
One approach in the art to remove this adhered formation material from the bit has been to provide nozzles in the bit body to direct drilling fluid from an interior plenum of the bit to the surface of the cutters. For example, in U.S. Pat. No. 4,883,132 to Tibbitts, to reduce bit balling, nozzles are provided that direct drilling fluid to impact the formation cuttings as they leave the cutting faces of the cutters. In some instances, however, the high velocity drilling fluid may not adequately remove the cuttings from the cutting elements. Moreover, the directed drilling fluid is not effective to remove cuttings from the bit face or junk slots of the bit.
The need to reduce frictional forces in the drilling process has been addressed in U.S. Pat. No. 4,665,996 to Foroulis et al. Foroulis discloses the application of a hard facing material to the surface of a drill pipe. The hard facing material is purported to reduce the friction between the drill string and the casing or rock. As a result, the torque needed for the rotary drilling operation, especially directional drilling, is decreased.
U.S. Pat. Nos. 5,447,208 and 5,653,300 to Lund et al. also disclose a way to reduce frictional forces associated with drilling, wherein the superabrasive cutting face of a cutting element is polished to a surface finish roughness of 10 xcexc in. or less.
There have been many instances in which a portion or all of certain drill bit and drilling tool surfaces have been coated with a layer of another material to promote wear resistance. For example, U.S. Pat. No. 5,163,524 to Newton et al. discloses application of a smooth, hard facing layer of an abrasion-resistant material to gage pads, the materials being suggested as suitable including a matrix material (WC) or a layer of CVD-applied xe2x80x9cpolycrytallinexe2x80x9d diamond. U.S. Pat. No. 4,054,426 to White suggests treating the surfaces of roller bit cones with a high particulate level ion plating process to form a dense, hard, smooth, thin film. U.S. Pat. No. 4,173,457 to Smith discloses hard facing of mining and drilling tools with sintered tungsten carbide-cobalt particles and with sintered or cemented chromium carbide particles. Of course, the use of tungsten carbide as a hard facing layer on drill bits has been known for decades, as disclosed in U.S. Pat. No. 2,660,405 to Scott et al., U.S. Pat. No. 2,883,638 to Owen and U.S. Pat. No. 3,301,339 to Pennebaker. Patterned hard facing on roller bit cones has been suggested in U.S. Pat. No. 5,291,807 to Vanderford et al., xe2x80x9ccarbidexe2x80x9d being suggested as a suitable material. Finally, U.S. Pat. No. 5,279,374 to Sievers et al. teaches the continuous or uninterrupted coating of rollercones carrying inserts with refractory material such as tungsten carbide.
None of the foregoing approaches to bit and cutter design, however, have specifically addressed the need to reduce frictional forces created by cuttings adhering to the bit body or bit components other than cutting elements. More specifically, the prior art has not addressed the effects of friction due to buildup of formation material at or proximate gaps, voids or other discontinuities created at interfaces between the cutters and the cutting face, the nozzles and the bit face, the roller-cone surfaces and inserts, or other points where parts of the bit are joined together or exterior surfaces of the bit join at sharp angles. Accordingly, it would be advantageous to provide a drill bit that reduces or eliminates adhesion of formation cuttings to the drill bit. It would also be advantageous to provide a method of treatment of at least selected portions of exposed surfaces of a bit that might be implemented on any drill bit regardless of shape, size or style.
The present invention provides a rotary-type drill bit for drilling subterranean formations and method of making the same. The bit according to the invention includes a surface treatment exhibiting relatively low adhesion for formation materials which extends over at least a portion of a bit surface exposed to drilling fluid. Advantages of such low-adhesion surface treatment of the invention include a reduction of bit balling, reduced frictional forces during the drilling process, and decreased erosion on the exposed surface of the drill bit.
In a more particular aspect of the invention, a nonwater-wet surface treatment comprised at least in part of a material such as an elastomer, plastic or precious metal or a superabrasive material, is applied to at least a portion of the exposed bit surface to prevent bit balling resulting from chemical bonds forming between hydrogen ions present in the clay unit layers of shale, as well as in other previously enumerated formations, and surfaces of the bit. Especially in areas on the bit face with low drilling fluid velocities thereover, such a treatment prevents the accumulation of cuttings, and consequent bit balling. Nonwater-wet surfaces do not possess hydrogen atoms to be shared with the formation material.
Also in accordance with the invention, a treatment material applied to the exposed bit surface may be polished, ground, lapped or otherwise processed by methods known in the art to create a smooth, low-adhesion surface which is also nonwater-wet.
Further in accordance with the invention, a surface treatment may comprise not only a treatment directly on a surface of a drill bit component but also a surface treatment on a surface of a preformed insert configured to provide such a surface treatment for a drill bit to which such insert is secured, or a preformed insert substantially, or even entirely, comprising a surface treatment material, the insert being secured to the drill bit component.
Advantages provided by a reduced roughness bit surface include increased rate of penetration because of reduced sliding frictional forces between the bit and the formation being drilled as well as reduced erosion of the bit and cutting elements (and particularly of the substrates and other carrier structures and the bit material adjacent pockets or apertures into which they are inserted). Furthermore, surface treatments according to the invention are easily applied to any shape, size or style of drill bit.
The foregoing and other features and advantages of the invention will become more readily apparent from the following detailed description of the preferred embodiments, which proceeds with reference to the drawings appended hereto.